Also, a day is a unit of time which usually lasts 86400 seconds, but can last 86401 or 86399 seconds to account for leap seconds (based on the UTC time scale).

"Birthday" referring to the whole day, then "day" referring to the time when the sun is shining light on Earth, and then struck through and replaced with "night" in reference to the fact that the sun was down by the time of my post.

Also, why is it important that a "day" lasts for 86400 seconds? That's just 1440 minutes, or 24 hours.